Oxygen and argon by back-pressured distillation

ABSTRACT

In a triple pressure cryogenic air distillation apparatus for producing of high purity oxygen and crude argon at low energy requirement, a novel method of avoiding proximity to argon freezeup conditions is disclosed. Referring to FIG. 3, the temperature at the overhead of the argon recovery column 309 is kept above about -305° F. by increasing the pressure of N 2  rejection column 304 to about 3 psi above normal discharge pressure. The exhaust N 2  is subsequently depressurized in one or more work-expanders 324 and 330 thereby producing process refrigeration.

DESCRIPTION

1. Technical Field

The invention relates to processes and apparatus for industrialseparation of air into oxygen of medium to high purity (>98%) and crudeargon byproduct by cryogenic distillation. The disclosed improvementachieves the high energy efficiency characteristic of modern tripledistillation pressure flowsheets while eliminating the argon freezeupproblem normally associated with these processes.

2. Background Art

There has been a continuing search for cryogenic air distillationflowsheets capable of producing high purity oxygen plus argon byproductat air supply pressures lower than the 95 psia (6.46 atmospheresabsolute) characteristic of conventional dual pressure columnflowsheets. The conventional dual pressure configuration with argonside-arm is illustrated by U.S. Pats. Nos. 2,934,908 and 2,934,907, plusthe journal article "Distillation of Air" by R. E. Latimer, ChemicalEngineering Progress Vol. 63 No. 2, February 1967, page 39.

If the nitrogen rejection column (the lower pressure column) of theconventional configuration is reboiled by condensing air rather thancondensing N₂, much lower air supply pressures are possible. However,high oxygen recovery requires a large amount of liquid nitrogen (LN₂)reflux at the top of the N₂ rejection column, hence most of the air mustgo to the HP rectifier to produce that LN₂. Similarly, high purityrequires high vapor rates through the argon stripper. These conflictingdemands were first satisfied by the flowsheet disclosed in U.S. Pat. No.3,688,513. In that flowsheet, the N₂ rejection column is reboiled by aminimum amount of condensing air, with the remainder being supplied tothe HP rectifier. The HP rectifier overhead provides some reboil to anintermediate height of the N₂ rejection column and also reboils a thirdcolumn which is at lower pressure than the N₂ rejection column. This LPcolumn is supplied from a sidestream liquid withdrawn from the N₂rejection column comprised of oxygen and argon. Argon stripping is doneat the bottom of both the LP column and the N₂ rejection column inproportion to the reboil through each stripping section.

Although both good recovery and good purity are obtained, this flowsheetunfortunately does not recover crude argon. Thus the 8 to 12% savings inenergy is offset by the lack of crude argon coproduct, which is usually5 to 10% of the total product value.

The reason the above triple pressure flowsheet cannot recover crudeargon is that the LP column only supplies reboil to the N₂ rejectioncolumn at a single height. The amount of reboil supplied from the HPcolumn directly to the N₂ rejection column intermediate height must belimited in order to obtain sufficient argon stripping reboil. Thusadditional N₂ rejection column reboil is necessary before muchconcentration and temperature change is incurred, to avoid "pinchingout". If the LP column effects the separation all the way from highpurity oxygen at the bottom to crude argon (>90%) at the top, itexperiences a temperature differential of 8° to 12° F. (4.5K to 6.7Kdifferential). Thus the top of such an argon (LP) column is not warmenough to provide N₂ rejection column reboil at the necessary height toprevent pinchout. The LP column of U.S. Pat. No. 3,688,513 flowsheetmust have low purity argon at the top, e.g., 20 to 50% purity, toprovide reboil adequately warm for the N.sub. 2 rejection column. Ifthis vapor were withdrawn as byproduct or as waste, it would represent aserious loss of oxygen product (lower recovery). Accordingly, theflowsheet incorporates a liquid recycle pump to return the LP columnoverhead product to the N₂ rejection column. Unfortuantely thisintroduces other undesirable consequences--the argon levels in bothcolumns are forced up by the recycle sufficiently to force most of theargon out of the N₂ rejection column overhead. This substantiallyincreases the required number of argon stripping trays in each column,and increases column pressure drops.

Thus there are two requirements which are very difficult to meetsimultaneously but both of which are essential to producing high purityoxygen at high recovery: sufficient LN₂ reflux and sufficient argonstripper reboil. Quantitatively, for every 100 moles of compressed airsupplied to the cold box, it is necessary to have at least about 30moles of N₂ overhead product at the HP rectifier and at least about 50moles of reboil through the argon stripper(s), in order to achieve atleast 95% recovery of available oxygen at at least 99.5% purity.

Both the method of producing process refrigeration and the method ofgasifying the product oxygen also can have substantial impact on theamount of available argon stripper reboil. In the above cited U.S. Pat.No. 3,688,513, refrigeration is produced by work-expanding about 13% ofthe supply air directly to the N₂ rejection column. This air providesneither argon stripper reboil nor HP rectifier N₂. The LOX is gasifiedat the base of the LP column, by latent heat exchange with HP rectifieroverhead N₂. Thus the fraction of supply air which ultimately gasifiesthe LOX is useful in producing LN₂, but not in argon stripping.

In U.S. Pat. No. 4,507,134, two modifications to the above triplepressure flowsheet are disclosed. First, crude argon is withdrawn fromthe LP column overhead by a vacuum compressor. As explained above, thisunavoidably means that either the crude argon has very large O₂ content,reducing recovery, or there must be a large transfer of reboil from theHP rectifier overhead to N₂ rejection column intermediate height,bypassing the LP column, which reduces recovery, purity, or both. Aneven greater difficulty is introduced by the means for LOX gasification:a fraction of the supply air is first partially condensed to reboil theN₂ rejection column bottom, and then the residue is totally condensed togasify the LOX. This produces a large quantity of quite impure LN₂,e.g., with 13% O₂ content, and greatly reduces the HP rectifier overheadN₂. Also the average O₂ content of the condensing gas which reboils theN₂ rejection column is somewhat lower, requiring a slightly higher airsupply pressure. The LOX is gasified at a somewhat higher pressure thanin the first flowsheet, and the LOX is pressurized by the conventionalpractice of using the hydrostatic head. However, the drawbacks incurredfor the higher O₂ pressure and crude argon withdrawal are greatlyreduced recovery plus possibly some loss in purity.

In copending U.S. Pat. No. 4,605,427 filed June 6, 1983 by the presentapplicant, a solution is disclosed to the problem of producing crudeargon while maintaining adequate reflux LN₂ and argon stripper reboil topermit high recovery at high purity. The solution is to have at leasttwo transfers of reboil from the LP column to the MP column, atvertically spaced tray heights. Thus the maximum amount of reboiltraverses the argon stripping section of the LP column, and then part istransferred to the MP column and only a reduced amount continues up theLP column to establish crude argon purity.

In addition to that basic and generally applicable disclosure, the aboveapplication also discloses a means for substantially increasing theargon stripper reboil. That is done by gasifying the LOX by latent heatexchange with LP column intermediate height vapor. Thus the supply airwhich ultimately gasifies the LOX contributes to both LN₂ refluxproduction and argon stripper reboil. The increase in argon stripperreboil is so substantial that the separate argon stripper at the base ofthe N₂ rejection column is no longer necessary, and hence lower airsupply pressure is possible. Also, much higher O₂ purities areobtainable. The drawback incurred is the lower O₂ delivery pressure, onthe order of 0.6 ATA.

In a second copending U.S. Pat. No. 4,578,095, filed Aug. 20, 1984 bythe present applicant, a triple pressure flowsheet is disclosed whichgasifies LOX at the HP rectifier overhead; has two argon strippingsections; and has at least two transfers of reboil from the LP column tothe N₂ rejection column, whereby crude argon of acceptable purity andrecovery (e.g., >80% purity and >50% recovery) is coproduced in additionto high purity oxygen at high recovery (>99.5% purity, >95% recovery).

In order to achieve full value from the crude argon byproduct, it shouldbe as pure as possible. Otherwise the purification cost is too high,consuming hydrogen and requiring added equipment. When the crude argonpurity of the three prior art disclosures described above is increased,a new problem arises. Pure liquid argon freezes at about -308.6° F., andalso at a pressure below 10.7 psia. The temperature at the overhead ofthe LP column is established by the temperature of that height of the N₂rejection column to which the reboil is transferred. Note that reboilcan be transferred either via indirect latent heat exchange with N₂rejection column liquid or via latent heat exchange with kettle liquidwhich is then fed into the N₂ rejection column. In a modern welldesigned configuration with N₂ rejection column overhead pressure andtemperature at about 17 psia and -318° F. respectively, the kettleliquid has a bubble temperature of -313° F., and the temperature of theN₂ rejection column height which receives reboil from the LP columnoverhead is in the -307° to -310° F. range. Even if the argon is keptabove 12 psia, corresponding to -306° F., local cold spots in thecondenser could cause localized freezing, which once started couldbecome progressively worse, causing serious upsets.

What is needed is a new process or apparatus which retains theadvantages of the flowsheets disclosed in U.S. Pat. Nos. 4,605,427 and4,578,095: high purity and recovery of both oxygen and crude argon, allat substantially reduced supply air pressure; but which avoids theunacceptable proximity to argon freezeup conditions. That is the primaryobjective of the newly disclosed process herein.

DISCLOSURE OF INVENTION

The disadvantages of the prior art are overcome by providing a triplepressure air distillation process or apparatus in which: the MP columnbottom is reboiled by partial or total condensation of at least part ofthe supply air; the LP column bottom is reboiled by HP rectifieroverhead product; at least one intermediate height of the MP columnreceives reboil from the LP column by at least one of latent heatexchange with MP column liquid or latent heat exchange with kettleliquid which is then fed to the MP column; product purity oxygen iswithdrawn from the LP column bottom, and crude argon is withdrawn fromthe LP column overhead; and in which the unique feature is the backpressuring of the MP column whereby overhead N₂ is withdrawn at apressure at least about 0.15 ATA (3 psi) above the normal withdrawalpressure and is work expanded to supply the required processrefrigeration.

Beyond the basic inventive entity as defined above or in the claims,numerous variations are possible in regard to particular featuresincorporated to achieve a desired product mix or conform to localconditions. For example, with the above described process the expanderflow does not bypass the HP rectifier, and hence increased LN₂ reflux isavailable. One means of capitalizing on this extra availability is togasify the LOX via condensing air in lieu of via HP rectifier overhead.This increases the O₂ delivery pressure. The liquid air thus madeavailable to reflux the MP column reduces the LN₂ requirement, such thatthe added increment obtained from what normally goes to the HP expanderis sufficient to retain full O₂ recovery.

Since essentially all of the N₂ is routed through the expander, only avery small pressure ratio is necessary. The MP column will be about 4 to5 psi higher than normal pressure, and the air supply pressure must beincreased by about 10 to 15 psi. This will still be about 15 psi lowerthan conventional high purity LOXBOIL plants require, and the O₂delivery pressure will be close to that found in LOXBOIL plants.

In other circumstances, where significant amounts of co-product highpurity N₂ are required, the preferred configuration would be to gasifythe LOX in the LP column bottom, i.e., withdraw it in gaseous phaserather than liquid phase, and use the extra LN₂ to achieve the requiredN₂ purity.

Even though crude argon is withdrawn, it will not necessarily be aproduct, depending on local market conditions, i.e., it may simply bevented. In the backpressured configuration, it is possible to keep theLP column overhead above atmospheric pressure, such that no argoncompressor is required for withdrawal. When routing to furtherpurification, it is normally further pressurized regardless ofwithdrawal pressure. The backpressured configuration raises all columntemperatures by about 3° F., providing that much added margin to argonfreezeup. All column pressures are about 20% higher, thus reducingcolumn volumes somewhat. The costs of these advantages are a physicallylarger expander and a higher air supply pressure compared tonon-backpressured triple pressure configurations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, the preferred or most representative embodiment of flowsheetwhich embodies the essential aspects of the disclosed invention, is asimplified flowsheet of a triple pressure air distillation arrangementincorporating backpressured expansion of MP column nitrogen; LOXBOIL bytotal condensation of supply air; two vertically spaced transfers ofreboil from LP to MP; two argon stripping sections; crude argonwithdrawal; plus a liquid air split which makes it possible for part ofthe liquid air to join the kettle liquid in refluxing the LP columnoverhead.

FIG. 2 illustrates a different location of the discharge N₂ expander anda triple transfer of reboil from LP to MP column.

FIG. 3 illustrates a different sensible heat exchange arrangement, adifferent liquid air split, a different expansion arrangement involvingtwo expanders, and a small latent heat exchange from HP rectifieroverhead to MP column intermediate height.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, air which has been compressed to a supply pressurenormally in the range of 4.6 to 5.6 ATA and cleaned and dried is cooledto near its dewpoint in main exchanger 101, at an example flowrate of1000 gram-moles/second ("m"). It is then split, with 260 m beingdirected to LOX gasifier 102 and the remainder to bottoms reboiler 103of N₂ rejection column (MP column) 104. Separator 105 removes thecondensed fraction and routes the remaining vapor to HP rectifier 106.The HP rectifier receives intermediate reflux from intermediate reboiler107 of the MP column. Overhead reflux is from reboiler 108 of LP column109. Kettle liquid from 105 and 106 is subcooled in sensible heatexchangers 110 and 111, then reduced in pressure in 112 and routed to LPcolumn overhead refluxer 113, where it is partially evaporated, then fedto MP column 104. LN₂ overhead from HP rectifier 106 is subcooled in111, reduced in pressure in 114, and after optional phase separation in115 the liquid is introduced as reflux into 104 overhead. Gaseous N₂ isexpanded in 116 and then provides cooling to incoming fluids insubcooler 111 and main exchanger 101 before being discharged, vented, orotherwise utilized (e.g., sieve regeneration). LP column 109 feed is aside withdrawal liquid stream from MP column 104 taken from just abovethe argon stripping section. Although the LP column pressure is slightlylower than that of the MP column, the column elevations and resultingliquid hydrostatic heads will normally require that pump 117 be used forthis transfer. Some subcooling is desirable, e.g., in subcooler 118,before feeding to 109. The liquid bottom from column 109 is warmed in118 and 110, combined with column 103 liquid bottom product, raised togasification pressure, and gasified in 102. Component 119, when present,may be the means for pressure adjustment and/or a hydrocarbon cleanupdevice such as molecular sieve. The elevations and hydrostatic headswill determine the need for check valves such as 120 to preventbackflooding of either column. The liquid air from 102 is subcooled in111 and then split by the combined action of means for pressurereduction 121 and 122. Sufficient liquid air is routed through 121 tomaintain both the desired temperature and liquid composition of thepartially evaporated stream exiting refluxer 113. Argon is withdrawnfrom column 109 overhead for further processing, and product puritygaseous O₂ is withdrawn from 102 and delivered through 101. Latent heatis exchanged from an intermediate height of LP column 109 above the feedpoint to an intermediate height of MP column 104 below the feed point bylatent heat exchanger 123.

As example and representative operating conditions for FIG. 1, 1000 mair enters 101 at 80 psia and exits at 78.5 psia. 260 m condenses at-285.2° F., gasifying 204 m of 99.5% O₂ at 25.3 psia. 135 m of theremainder condenses in 103, causing 117 m reboil up the MP argonstripper. The remaining 605 m of uncondensed air is routed to the HPrectifier 106, having overhead pressure 76.5 psia. 298 m of kettleliquid is combined with 135 m liquid from 105 and routed to 113. 307 mof LN₂ is routed via 114 to column 104 overhead. Column 104 has bottompressure of 26.5 psia, overhead pressure of 22.3 psia, and overheadtemperature of -313.6° F. 260 m liquid air from 102 is split and partrouted through 121 so as to maintain the partially evaporated fluidexiting 113 at -304° F. and at 50% O₂ in the liquid phase. LP column 109has 20 psia bottom pressure 15.2 psia overhead pressure at -301.3° F.,and 6.1 m of crude argon at 92% purity is withdrawn overhead. Thebackpressured N₂ drops about 8° F. in temperature through the expander,and helps minimize LN₂ flashing by providing full subcooling. The MPcolumn reboil is increased by about 90 m at 107, and the LP columnreboil is decreased by about 140 m at 123. 154 m of liquid oxygen iswithdrawn from 109, and combined with 50 m withdrawn from 104 forgasification. 160 m of sidestream liquid O₂ containing about 4.3% argonis pumped by 117.

In FIGS. 2 and 3 components having function or description correspondingto components of FIG. 1 have the same number in the 200 series or 300series respectively, and will not be further described.

In FIG. 2, expander 224 is located at a warmer location thancorresponding expander 116. This requires lower pressure ratio drop, butresults in more loss of LN₂ to flashing. Three latent heat exchanges arepictured between the LP column and MP column, with latent heat exchanger225 providing the new third one. This largely eliminates the advantagesfrom a liquid air split, which accordingly is eliminated. LOX pump 226is pictured as it might be required if hydrostatic head is insufficientto pressurize MP column liquid bottom product.

In FIG. 3, the main exchanger is illustrated having two cores, 301 and327. Two expanders, 324 and 330, operate at different temperatures. MPintermediate height reboil is supplied from HP rectifier overhead viceintermediate height, via exchanger 328. Optional pressure control valve329 guards against excessive depressurization of 313. Liquid air issplit into two direct injection streams, one being pumped to the HPrectifier via pump 331 and the other conventionally to the MP column. Adifferent subcooling arrangement is pictured, incorporating exchanger332. MP column bottom liquid withdrawal is controlled by valve 333.

Obviously all possible combinations of desirable features have not beenpictured, but rather only a few representative ones. LOX can be gasifiedat the LP column bottoms, by HP rectifier overhead, or by LP columnintermediate height liquid. The argon stripping section of the MP columncan be omitted, with bottom liquid routed to the LP column. Additionalembodiments are possible within the scope of the claimed invention.

I claim:
 1. A process for producing oxygen of at least about 98% purity and optionally also crude argon from air at a supply pressure of between about 4.6 and 5.6 ATA in a triple pressure distillation apparatus comprised of a high pressure (HP) rectifier, a medium pressure (MP) nitrogen rejection column, and a low pressure (LP) argon recovery column, comprising:(a) at least partially condensing at least part of the supply air to supply bottom reboil to the MP column; (b) exchanging latent heat from the HP rectifier overhead to the LP column bottoms; (c) exchanging latent heat from the LP column to at least one intermediate height of the MP column; (d) withdrawing gaseous N₂ overhead product from the MP column at a pressure which is at least about 0.15 ATA above the discharge pressure; (e) work expanding said gaseous N₂ product; and (f) withdrawing product purity oxygen from the LP column bottoms, and crude argon from the LP column overhead.
 2. Process according to claim 1 further comprising stripping argon from product purity O₂ liquid in both the MP column and LP column bottom sections, and transferring liquid sidestream from the MP column above the argon stripping section to the LP column.
 3. Process according to claim 1 further comprising transferring MP column liquid bottom product to the LP column, and stripping argon from product purity lqiuid oxygen only in the LP column.
 4. Process according to claim 1 further comprising gasifying product purity liquid oxygen by latent heat exchange with a totally condensing fraction of the supply air.
 5. Process according to claim 4 further comprising splitting the liquid air, supplying part to reflux an MP column intermediate height, and combining the remainder with kettle liquid for refluxing the LP column overhead.
 6. Process according to claim 1 further comprising gasifying product purity liquid oxygen by latent heat exchange with partially condensing air.
 7. Process according to claim 1 further comprising gasifying product purity liquid oxygen by latent heat exchange with HP rectifier overhead vapor.
 8. Process according to claim 1 further comprising exchanging latent heat from HP rectifier intermediate height to MP column intermediate height which is below the feed point.
 9. Process according to claim 1 further comprising exchanging latent heat from an intermediate height of the LP column above the argon stripping section to an intermediate height of the MP column.
 10. Process according to claim 1 further comprising compressing supply air to a pressure between 4.6 and 5.6 ATA, withdrawing oxygen of at least 99% purity and 94% recovery at a pressure of at least 1.4 ATA, and recovering at least 50% of the argon at at least 80% purity.
 11. Apparatus comprising means designed for distilling air to oxygen of at least 98% purity including:a. HP rectifier; b. MP column which is supplied liquid N₂ overhead reflux from the HP rectifier overhead, and with means for N₂ vapor withdrawal overhead; c. at least one N₂ expander for maintaining a backpressure on the MP column, said expander being connected to said means for N₂ vapor withdrawal; d. LP column including bottoms reboiler supplied with heat from the HP rectifier overhead vapor; e. means for refluxing LP column overhead by at least one ofi. latent heat exchange with kettle liquid from said HP rectifier or ii. latent heat exchange with MP column intermediate height liquid; f. means for withdrawing crude argon from LP column overhead; and g. reboiler for MP column bottom liquid which is supplied latent heat from partial condensation of the supply air.
 12. Apparatus according to claim 11 further including means for exchanging latent heat from an intermediate height of the LP column to an intermediate height of the MP column.
 13. Apparatus according to claim 12 further including argon stripping sections in the bottom sections of both of said LP and MP columns, and means for transferring sidestream liquid from said MP column to said LP column.
 14. Apparatus according to claim 12 further including means for gasifying liquid oxygen by total condensation of a fraction of the supply air.
 15. Apparatus according to claim 14 further including means to split said condensed air into one stream which is directly injected into said MP column and a second stream which is used for indirect refluxing of said LP column.
 16. Apparatus according to claim 12 further including means to split the N₂ withdrawn from the MP column and a second expander for the split stream which oeprates at a different temperature than said first expander. 